1. Field of the Invention
The present invention relates to an electric cable comprising a conductor element, and successively around said conductor element: an electrically-insulating layer; a metal screen; and an outer protective sheath.
The invention applies typically, but not exclusively, to the fields of high or very high voltage alternating current (AC) or direct current (DC) power cables. Such power cables are typically 60 kilovolt (kV) to 600 kV cables.
2. Background of the Invention
High or very high voltage power cables typically comprise a central conductor element and, successively and coaxially around said conductor element: an inner semiconductive screen; an extruded electrically-insulating layer; an outer semiconductive screen; a metal screen; and an outer protective sheath. The outer protective sheath is usually made out of materials that retard or withstand flame propagation. The sheath may be of the halogen-free flame-retardant (HFFR) type.
It has been observed that while such electric cables are being installed, more particularly while such cables are being placed or while junctions and terminations are being laid on such cables, it is possible for the outer protective sheath to be damaged, thereby creating a premature defect in the outer sheath that will lead to the cable deteriorating, in particular by moisture penetrating into said cable.
In order to verify whether the electric cable has been damaged while being installed, it is known to perform an electric test using a high voltage of constant polarity (DC) between the metal screen and a conductive covering deposited on said outer protective screen. The test may also be repeated throughout the lifetime of the electric cable.
A first technique consists in coating the outer protective sheath of the electric cable in a layer of graphite in powder form. Nevertheless, graphite powder is difficult to handle and runs the risk of dirtying sheathing workshops. Furthermore, it is difficult to distribute graphite powder uniformly over the entire periphery of the outer protective sheath of the electric cable because the adhesion of graphite powder on said outer sheath is not strong. Such non-uniformity in the layer of graphite on the outer protective sheath means that said electric test cannot be carried out reliably.
A second technique consists in applying a conductive varnish on the outer protective sheath of the electric cable. The disadvantage of that technique is the presence of volatile solvents in the conductive varnish that may be irritating and/or toxic. Furthermore, said varnish possesses mechanical properties that are very different from those of the outer protective sheath, which may compromise good adhesion of the varnish on the outer protective sheath while the electric cable is being handled.